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We are getting high fecal coliform results despite having good (>2 mg/l) chlorine residuals. I'd like to know the affect ammonia, nitrite and/ or nitrate has on the disinfecting ability of hypochlorite. We are not seeing a very high chlorine demand which (about 2-3 mg/l). We know that nitrite acts as a chlorine sponge and is present during partial nitrification but we don't think that is happening. Does anyone have any experience with this occurrence?

My experience is that chlorine dosage and contact time are what determines disinfection. Is your dosage the same? Is the contact time (flow) the same?

If your effluent contains any significant organic nitrogen compounds then there are competing reactions between the organic nitrogen and chlorine making different nitrogen-chlorine compounds that are indicated as total chlorine. These different compounds vary a lot as to disinfection capabilities.

If your flows have increased then you might need to feed a higher clorine dose to maintain disinfection due to the reduction in contact time.

Hypo could be loosing its strength if stored to long. Hypochlorite looses strength quite papidly it has a relatively short shelf life. You may have to increase the dosage rate and or increase Blower Amps. to supply more air to supplement oxidation.

Check for free chlorine. If you have 2mg/l total chlorine but no free, I would expect that there are chloramines being made. Chloramines don't disinfect as well as chlorine and show up as total chlorine.

You don't say anthing about your treatment system. Any chance of contamination of your treated effluent by runoff, presence of geese at the discharge etc.?

We had an occasion at a small plant, lagoon with ave flow of 25K gpd, where we had high levels of chlorine residual 1.5 to >2.0 and were still seeing sporadic high fecals. Traced it to geese roosting/standing at the discharge flume where we sampled. We put a cover over the trench, problem went away.